"One of the major questions about the extinct megafauna is what caused them to suddenly go extinct all around the world," says Cooper. "Is it humans or climate change or some combination of both?"

Cooper and colleagues investigated this question in New Zealand by analysing DNA from ancient moa bones.

The moa stood up to 2.5 metres high, weighed 250 kilograms, and was the largest herbivore in New Zealand prior to the arrival of Polynesian settlers around 1280 AD, when it died out.

"With the bones from New Zealand we were able to study the genetics of the population before, during and after human arrival," says Cooper. "We could track them moving around the country through time and space."

The research team sampled and dated bones from several populations of two closely related species of moa from sites all over New Zealand.

One species was cold-adapted, the other was warm-adapted and each had their own ecological niche.

The bones sampled dated back into the Pleistocene period, when there were repeated glaciations, and into the current stable Holocene period.

"So we can track the behaviour of both of them as the temperature went up and down," says Cooper.

The researchers also tracked how the size of the moa populations changed over time by measuring their change in genetic diversity.

"You can calculate the population size reasonably well using genetic sequences from a large number of individuals," says Cooper. "There's not much diversity if the population is small."

The researchers found no evidence that the moa population suffered significantly during the climate change of the past 40,000 years.

"They adapted by shifting around to find new conditions that suited them," says Cooper.

But, 700 years ago, despite a relatively stable climate, within a space of 50 years the birds went extinct.

"The population was not doing badly before humans arrived and then suddenly it was gone shortly thereafter," says Cooper.

"This study shows that in the absence of humans, the New Zealand avian megafauna were able to respond to climate and environmental change and that the likelihood of their extinction due to the effects of such changes alone was low," write Cooper and colleagues in their paper.

The conclusion is that overhunting and habitat destruction by humans was to blame.

The researchers are also analysing DNA and plant remains in moa dung as old as 6000 years to see what each species ate.

"What you have in New Zealand is a potential to reconstruct the whole ecosystem of megafauna before humans using dung and bone," says Cooper. "You can't do that anywhere else in the world."